把二叉树打印成多行

题目描述：从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

 

public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}

思路一：

import java.util.ArrayList;import java.util.LinkedList;public class Solution {    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null)            return res;        LinkedList<TreeNode> queue = new LinkedList<>();        queue.add(pRoot);        while (!queue.isEmpty())        {            int start = 0;            int end = queue.size();            ArrayList<Integer> list = new ArrayList<>();            while (start++ < end)            {                TreeNode node  = queue.poll();                if (node != null)                    list.add(node.val);                if (node.left != null)                    queue.add(node.left);                if (node.right != null)                    queue.add(node.right);            }            res.add(list);        }        return res;    }}

另一种写法：

import java.util.ArrayList;import java.util.LinkedList;public class Solution {    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null)            return res;        LinkedList<TreeNode> queue = new LinkedList<>();        queue.add(pRoot);        ArrayList<Integer> list =new ArrayList<>();        int start = 0;        int end = 1;        while (!queue.isEmpty())        {            TreeNode node = queue.poll();            start++;            if (node != null)                list.add(node.val);            if (node.left != null)                queue.add(node.left);            if (node.right != null)                queue.add(node.right);            if (start == end)            {                if (!list.isEmpty())                    res.add(list);                start = 0;                end = queue.size();                list = new ArrayList<>();            }        }        return res;    }}

思路二：

import java.util.ArrayList;import java.util.Iterator;import java.util.LinkedList;public class Solution {    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null)            return res;        LinkedList<TreeNode> queue = new LinkedList<>();        queue.add(null);        queue.add(pRoot);        ArrayList<Integer> list =new ArrayList<>();        while (queue.size() > 1)        {            TreeNode node = queue.poll();            if (node == null)            {                Iterator<TreeNode> iter = queue.iterator();                while (iter.hasNext())                    list.add(iter.next().val);                res.add(new ArrayList<>(list));                list.clear();                queue.add(null);                continue;            }            if (node.left != null)                queue.add(node.left);            if (node.right != null)                queue.add(node.right);        }        return res;    }}

思路三：

两个队列交替打印

import java.util.ArrayList;import java.util.LinkedList;public class Solution {    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null)            return res;        LinkedList<TreeNode> q1 = new LinkedList<>();        q1.add(pRoot);        LinkedList<TreeNode> q2 = new LinkedList<>();        while (!q1.isEmpty() || !q2.isEmpty())        {            ArrayList<Integer> list = new ArrayList<>();            while (!q1.isEmpty())            {                TreeNode tmp = q1.poll();                if (tmp != null)                    list.add(tmp.val);                if (tmp.left != null)                    q2.add(tmp.left);                if (tmp.right != null)                    q2.add(tmp.right);            }            if (!list.isEmpty())            {                res.add(list);                list = new ArrayList<>();            }            while (!q2.isEmpty())            {                TreeNode tmp2 = q2.poll();                if (tmp2 != null)                    list.add(tmp2.val);                if (tmp2.left != null)                    q1.add(tmp2.left);                if (tmp2.right != null)                    q1.add(tmp2.right);            }            if (!list.isEmpty())            {                res.add(list);                list = new ArrayList<>();            }        }        return res;    }}

思路四：递归

import java.util.ArrayList;public class Solution {    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        if (pRoot == null)            return res;        depth(pRoot, 1, res);        return res;    }    private void depth(TreeNode root, int depth, ArrayList<ArrayList<Integer>> lists)    {        if (root == null)            return;        if (depth > lists.size())            lists.add(new ArrayList<>());        lists.get(depth - 1).add(root.val);        depth(root.left, depth + 1, lists);        depth(root.right, depth + 1, lists);    }}